1. Field of the Invention
This invention relates to novel amphoteric starch derivatives and their application as wet-end additives for improving both wet and dry strength of paper.
2. Description of the Prior Art
Commercial wet-strength paper additives, which include polyamide-polyamine-epichlorohydrin (PAE), urea-formaldehyde (UF), malamine-formaldehyde (MF), polyethylenimine (PEI), modified polyacrylamide, and similar resins, are derived from diminishing reserves of petroleum and natural gas products and/or high energy processes. They are thereby rapidly becoming economically impractical. Moreover, these resins generally cause difficulty in paper repulping processes and are not readily biodegradable, thereby being ecologically undesirable.
In some cases, neutral to alkaline pH papermaking systems are required to improve properties such as interfiber bonding, stability of paper on aging, retention of alkaline fillers, and paper softness. However, except for PAE, the above-mentioned commercial wet-strength resins function marginally unless the pH of the paper-making systems are in the range of about 4.5-5.5. These acidic conditions of course contribute to equipment corrosion and increased maintenance costs.
The principal industrial wet-end dry-strength paper additives are the cationic starches. Exemplary of these are the starch ethers containing tertiary and quaternary amines as taught in U.S. Pat. Nos. 2,813,093 (Caldwell et al.) and 2,876,217 (Paschall), and also sulfonium and phosphonium starch derivatives as described in U.S. Pat. Nos. 2,989,520 (Rutenberg et al.) and 3,077,469 (Aszalos), respectively. Cationic starches do not improve paper wet strength, and frequently their effectiveness is significantly reduced when furnish pH is appreciably above 5.5, particularly for the tertiary amines.
When both wet- and dry-strength additives are required in commercial paper, it is often necessary to use a dry-strength agent, such as a cationic starch, and a wet-strength resin. As a consequence of interaction between the resin and the cationic starch, the efficiency of both additives is significantly reduced.
In U.S. Pat. No. 3,763,060 (Hamerstrand et al.) is disclosed the wet-end addition to paper of interpolymers such as starch xanthate crosslinked with PAE resin, and U.S. Pat. No. 3,436,305 (Maher) teaches starch xanthate crosslinked with PEI. though these compositions are somewhat effective for the purpose of strengthening paper, they suffer from the disadvantages characteristic of the resinous components as described above.
Other additives have been designed which have bi- or mult-functional activity, such as dry-strength improvement and pigment or filler retention. Predominant in this field are the amphoteric strengthening agents such as the polysalt coacervates of U.S. Pat. No. 3,790,514 (Economou) and the polysaccharides having both cationic and anionic substituents. This latter group is the subject of U.S. Pat. Nos. 3,467,647 (Benninga), 3,459,632 (Caldwell et al.), 3,649,624 (Powers et al.), 3,793,310 (Elizer), and 3,562,103 (Moser et al.). Starch is the most commonly used backbone of these substituted polysaccharide compositions. The cationic groups are usually the tertiary and quaternary amines, and others such as sulfonium and phosphonium groups have also been used. Typical of the anionic substituents are phosphates, phosphonates, carboxylates, sulfates, and sulfonates. Though all of these amphoteric compositions have at least some degree of paper dry-strength properties, they have not been observed to impart to paper significant wet strength.
Certain polysaccharide xanthates, such as the ammonium cellulose xanthates of Bridgeford et al., U.S. Pat. No. 3,336,144, are known to impart wet strength to paper. Such additives have not been widely accepted in the paper industry because they operate in a relatively narrow pH range and have no effect on dry strength. When starch is substituted for cellulose as within the scope of Bridgeford and the resultant composition is used as a wet-end additive, no increase in wet strength is observed.